2,805 research outputs found
Online Fashion Magazines
Fashion magazines have been a staple in the American home for many years. As the digital age continues, however, many magazines are choosing to include technology and offer digital copies of their magazines. In Bethany Potten’s presentation, Online Fashion Magazines, we learned that the introduction of digital magazines will not destroy the print world, but enhance it
Semiconservative quasispecies equations for polysomic genomes: The general case
This paper develops a formulation of the quasispecies equations appropriate
for polysomic, semiconservatively replicating genomes. This paper is an
extension of previous work on the subject, which considered the case of haploid
genomes. Here, we develop a more general formulation of the quasispecies
equations that is applicable to diploid and even polyploid genomes.
Interestingly, with an appropriate classification of population fractions, we
obtain a system of equations that is formally identical to the haploid case. As
with the work for haploid genomes, we consider both random and immortal DNA
strand chromosome segregation mechanisms. However, in contrast to the haploid
case, we have found that an analytical solution for the mean fitness is
considerably more difficult to obtain for the polyploid case. Accordingly,
whereas for the haploid case we obtained expressions for the mean fitness for
the case of an analogue of the single-fitness-peak landscape for arbitrary
lesion repair probabilities (thereby allowing for non-complementary genomes),
here we solve for the mean fitness for the restricted case of perfect lesion
repair.Comment: 16 pages, 3 figure
Evolutionary dynamics of adult stem cells: Comparison of random and immortal strand segregation mechanisms
This paper develops a point-mutation model describing the evolutionary
dynamics of a population of adult stem cells. Such a model may prove useful for
quantitative studies of tissue aging and the emergence of cancer. We consider
two modes of chromosome segregation: (1) Random segregation, where the daughter
chromosomes of a given parent chromosome segregate randomly into the stem cell
and its differentiating sister cell. (2) ``Immortal DNA strand''
co-segregation, for which the stem cell retains the daughter chromosomes with
the oldest parent strands. Immortal strand co-segregation is a mechanism,
originally proposed by Cairns (J. Cairns, {\it Nature} {\bf 255}, 197 (1975)),
by which stem cells preserve the integrity of their genomes. For random
segregation, we develop an ordered strand pair formulation of the dynamics,
analogous to the ordered strand pair formalism developed for quasispecies
dynamics involving semiconservative replication with imperfect lesion repair
(in this context, lesion repair is taken to mean repair of postreplication
base-pair mismatches). Interestingly, a similar formulation is possible with
immortal strand co-segregation, despite the fact that this segregation
mechanism is age-dependent. From our model we are able to mathematically show
that, when lesion repair is imperfect, then immortal strand co-segregation
leads to better preservation of the stem cell lineage than random chromosome
segregation. Furthermore, our model allows us to estimate the optimal lesion
repair efficiency for preserving an adult stem cell population for a given
period of time. For human stem cells, we obtain that mispaired bases still
present after replication and cell division should be left untouched, to avoid
potentially fixing a mutation in both DNA strands.Comment: 9 pages, 3 figure
Keratinocyte Stem Cells: a Commentary1
For many years it has been widely accepted that stem cells play a crucial role in adult tissue maintenance. The concept that the renewing tissues of the body contain a small subcompartment of self-maintaining stem cells, upon which the entire tissue is dependent, is also now accepted as applicable to all renewing tissues. Gene therapy and tissue engineering are driving considerable interest in the clinical application of such hierarchically organized cellular compartments. Recent initial observations have provided a tantalizing insight into the large pluripotency of these cells. Indeed, scientists are now beginning to talk about the possible totipotency of some adult tissue stem cells. Such work is currently phenomenologic, but analysis of data derived from genomics and proteomics, identifying the crucial control signals involved, will soon provide a further impetus to stem cell biology with far reaching applications. The epidermis with its relatively simple structure, ease of accessibility, and the ability to grow its cells in vitro is one obvious target tissue for testing stem cell manipulation theories. It is crucial, however, that the normal keratinocyte stem cell is thoroughly characterized prior to attempting to manipulate its pluripotency. This commentary assesses the data generated to date and critically discusses the conclusions that have been drawn. Our current level of understanding, or lack of understanding, of the keratinocyte stem cell is reviewed
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